News Release Number: STScI-2004-34

Stellar Survivor from 1572 A.D. Explosion Supports Supernova Theory

The full news release story:

An international team of astronomers is announcing today that they have
identified the probable surviving companion star to a titanic supernova
explosion witnessed in the year 1572 by the great Danish astronomer
Tycho Brahe and other astronomers of that era.

This discovery provides the first direct evidence supporting the
long-held belief that Type Ia supernovae come from binary star systems
containing a normal star and a burned-out white dwarf star. The normal
star spills material onto the dwarf, which eventually triggers an
explosion.

The results of this research, led by Pilar Ruiz-Lapuente of the
University of Barcelona, Spain, are being published in the Oct. 28
British science journal Nature. "There was no previous evidence pointing
to any specific kind of companion star out of the many that had been
proposed. Here we have identified a clear path: the feeding star is
similar to our Sun, slightly more aged," Ruiz-Lapuente says. "The high
speed of the star called our attention to it," she added.

Type Ia supernovae are used to measure the history of the expansion rate
of the universe and so are fundamental to helping astronomers understand
the behavior of dark energy, an unknown force that is accelerating the
expansion of the universe. Finding evidence to confirm the theory as to
how Type Ia supernovae explode is critical to assuring astronomers that
the objects can be better understood as reliable calibrators of the
expansion of space.

The identification of the surviving member of the stellar duo reads like
a crime scene investigation tale. Even though today's astronomers
arrived at the scene of the disaster 432 years later, using astronomical
forensics they have nabbed one of the perpetrators rushing away from the
location of the explosion (which is now enveloped in a vast bubble of
hot gas called Tycho's Supernova Remnant). For the past seven years the
runaway star and its surroundings were studied with a variety of
telescopes. The Hubble Space Telescope played a key role by precisely
measuring the star's motion against the sky background. The star is
breaking the speed limit for that particular region of the Milky Way
Galaxy by moving three times faster than the surrounding stars. Like a
stone thrown by a sling, the star went hurtling off into space,
retaining the velocity of its orbital motion when the system was
disrupted by the white dwarf's explosion.

This alone is only circumstantial evidence that the star is the perpetrator
because there are alternative explanations to its suspicious behavior.
It could be falling in at a high velocity from the galactic halo that
surrounds the Milky Way's disk. But spectra obtained with the 4.2-meter
William Herschel Telescope in La Palma and the 10-meter W.M. Keck
telescopes in Hawaii show that the suspect has the high heavy-element
content typical of stars that dwell in the Milky Way's disk, not the halo.

The star found by the Ruiz-Lapuente team is an aging version of our Sun.
The star has begun to expand in diameter as it progresses toward a
red-giant phase (the end stage of a Sun-like star's lifetime). The star
turns out to fit the profile of the perpetrator in one of the proposed
supernova conjectures. In Type Ia supernova binary systems, the more
massive star in the pair will age faster and eventually becomes a white
dwarf star. When the slower-evolving companion star subsequently ages to
the point where it begins to balloon in size, it spills hydrogen onto the
dwarf. The hydrogen accumulates until the white dwarf reaches a critical
and precise mass threshold, called the Chandrasekhar limit, where it
explodes as a titanic nuclear bomb. The energy output of this explosion is
so well known that it can be used as a standard candle for measuring vast
astronomical distances. (An astronomical "standard candle" is any type of
luminous object whose intrinsic power is so accurately determined that it
can be used to make distance measurements based on the rate the light dims
over astronomical distances).

"Among the various systems containing white dwarfs that receive material
from a solar-mass companion, some are believed to be viable progenitors
of Type Ia supernovae, on theoretical grounds. A system called U Scorpii
has a white dwarf and a star similar to the one found here. These results
would confirm that such binaries will end up in an explosion like the one
observed by Tycho Brahe, but that would occur several hundreds of thousands
of years from now," says Ruiz-Lapuente.

An alternative theory of Type Ia supernovae is that two white dwarfs
orbit each other, gradually losing energy through the emission of
gravitational radiation (gravity waves). As they lose energy, they
spiral in toward each other and eventually merge, resulting in a white
dwarf whose mass reaches the Chandrasekhar limit, and explodes. "Tycho's
supernova does not appear to have been produced by this mechanism, since
a probable surviving companion has been found," says Alex Filippenko of
the University of California at Berkeley, a co-author on this research.
He says that, nevertheless, it is still possible there are two different
evolutionary paths to Type Ia supernovae.

On November 11, 1572, Tycho Brahe noticed a star in the constellation
Cassiopeia that was as bright as the planet Jupiter (which was in the
night sky in Pisces). No such star had ever been observed at this
location before. It soon equaled Venus in brightness (which was at -4.5
magnitude in the predawn sky). For about two weeks the star could be
seen in daylight. At the end of November it began to fade and change
color, from bright white to yellow and orange to faint reddish light,
finally fading away from visibility in March 1574, having been visible
to the naked eye for about 16 months. Tycho's meticulous record of the
brightening and dimming of the supernova now allows astronomers to
identify its "light signature" as that of a Type Ia supernova.

Tycho Brahe's supernova was very important in that it helped
16th-century astronomers abandon the idea of the immutability of the
heavens. At the present time, Type Ia supernovae remain key players in
the newest cosmological discoveries. To learn more about them and their
explosion mechanism, and to make them even more useful as cosmological
probes, a current Hubble Space Telescope project led by Filippenko is
studying a sample of supernovae in other galaxies at the very time they
explode.